Asynchronous Mapping Procedure (AMP) Justification for Mapping and

Frequency justification in OTN is required for some of the CBR mapping techniques and for TDM multiplexing. As indicated in Table 5, the AMP justification technique is a hybrid of the techniques used for asynchronous/PDH networks and SONET/SDH. Similar to the PDH networks, the justification is based on an asynchronous technique with justification control fields rather than the pointer-based approach of SONET/SDH. Like SONET/SDH, however, it provides for both positive and negative byte-wise adjustments rather than the bit-oriented positive adjustments of PDH.

21 The jitter and wander requirements for OTN network interfaces are specified in ITU-T G.8251.

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Table 5 Comparison of PDH, SONET/SDH, and OTN AMP frequency justification Hierarchy Technique Adjustment increment

PDH Positive justification (stuff) Single bit SONET /

SDH

Positive/negative/zero (pnz) justification (via pointers)

Single byte for SONET VTs and STS-1 (SDH VC-1/2/3). N bytes for SONET STS-Nc, 3 bytes for SDH VC-4, and 3N bytes for SDH VC-4-Nc.

OTN Positive/negative/zero justification

Single byte

The justification overhead (JOH) in the OTN is the Justification Control (JC), Negative Justification Opportunity (NJO), and Positive Justification Opportunity (PJO) bytes. As illustrated in Figure 7, these bytes are part of the OPUk overhead. The NJO provides a location for inserting an additional data byte if the client signal is delivering data at a faster rate than the OPUk payload area can accommodate. The PJO provides a stuff opportunity if the client signal is delivering data a lower rate than the OPUk payload area can accommodate. The NJO is thus analogous to the SONET/SDH H3 byte and the PJO to the SONET/SDH positive stuff

opportunity byte. The demapper ignores the contents of the NJO or PJO bytes whenever they carry a justification byte. Bits 7 and 8 of JC are used to indicate the contents of the NJO and PJO, somewhat analogous to the SONET/SDH H1 and H2 or the PDH C-bits. The mapper assigns the same value to each of the three JC bytes in an OPUk frame so that the demapper can perform a two-of-three majority vote for error correction.

AMP Mapping Frequency Justification

Table 6 shows the definitions of JC, NJO, and PJO for the AMP and BMP CBR mappings.

Here, for the mapping application, PJO is a single byte in the OPUk payload area. As noted in section 5.1, justification is only required for asynchronous mapping since the OPUk clock is generated independently of the client signal clock. Since the bit-synchronous mapping uses an OPUk clock derived from the client signal, it can use fixed assignments for NJO and PJO.

Table 6 Justification control and opportunity definitions for AMP and BMP CBR mappings

JC [78]

Generation by AMP mapper Generation by BMP mapper

10 not generated justification

byte

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AMP Multiplexing Frequency Justification

The AMP justification multiplexing frame format differs somewhat from the mapping frame format of Figure 7. First, in addition to an NJO, the ODTUjk includes two PJO bytes rather than a single PJO. This structure easily accommodates the ±20 ppm of the LO ODU signals.

The JOH interpretation for the use of the NJO, PJO1, and PJO2 is shown in Table 7.

Table 7 Justification control and opportunity definitions for TDM with AMP JC

[78]

NJO PJO1 PJO2 Interpretation by the demapper

00 justification byte

data byte data byte no justification (0)

01 data byte data byte data byte negative justification (-1) 10 justification

byte

justification byte

justification byte

double positive justification (+2)

11 justification byte

justification byte

data byte positive justification (+1)

The second frame modification is to accommodate the frequency justification for multiple LO ODU tributaries. Each LO ODU tributary that is being multiplexed into the HO OPU requires its own justification. Since there is only a single set of JC bytes and NJO byte in each OPUk frame, they must be shared among the tributary ODUs. This sharing is done based on the frame number within the multiframe. Figure 17 illustrates this sharing of the OPUk JOH overhead.

As can be seen, the number of justification opportunities for client signal in each HO OPU multiframe is equal to the number of TS that the client signal uses.

When a client signal is mapped into an OPUk, the PJO is always located in the fourth row of the first payload column (column 17), as illustrated in Figure 7. The PJO locations continue to use the fourth row of the HO ODU in the multiplexing structure. However, when signals are multiplexed into an OPUk with AMP, the PJO locations must be located within a TS column associated with that client. For that reason, the first PJO (PJO1) is located in the lowest numbered column used by that client signal and the second PJO (PJO2) is located in the next column used by that client. Further, the NJO, PJO1 and PJO2 bytes for a client signal appear during each frame in the HO ODU multiframe where the frame number is equal to the number of a TS used by that client. The PJO1 and PDO2 locations are illustrated in the examples of Figure 17.

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Figure 17 illustrates an example of a LO ODU2 that uses TS # 4, 5, 8 and 10 when it is multiplexed into an OPU3. Its NJO and PJO bytes appear in frames 4, 5, 8 and 10 of the 16-frame ODU3 multi16-frame, as indicated by the MFAS LSB values 3, 4, 7 and 9, respectively. In each of these four frames, PJO1 is located in column 20, since that is the first OPU3 payload column associated with the lowest number TS used by that ODU2²². The PJO2 is located in column 21 since that is the first payload column associated with the next lowest TS used by that client. Similarly, if an ODU2 is multiplexed into eight 1.25G TS of an OPU3, its JOH, NJO, and PJO bytes appear in the eight frames of the 32-frame ODU3 multiframe corresponding to the TS that it uses.

It is important to note that while the SONET JOH bytes are located within the transport

overhead, the OTN JOH bytes are located within the OPUk overhead, which is analogous to the SONET Path overhead. This JOH location choice has an important implication: Retiming an OTN signal requires demultiplexing back to the client signal.

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Figure 17 Examples showing the AMP Justification Opportunity byte locations

16

PJO1 PJO1 PJO1 PJO1 PJO2 PJO2 PJO2 PJO2 17 18 19MFAS

bits 5678 0000

0001

0010

PJO1 PJO1 PJO1 22 23 24

PJO2

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6.2.2 Bit-synchronous Mapping Procedure (BMP) Justification for Mapping